[GPUModelRunner] Split code related to kv cache init to a separate file

[heheda12345]

Purpose

The function GPUModelRunner.initialize_kv_cache and its dependency are too long. Split it to a separate file.

Add myself to codeowner of this file.

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request refactors the GPUModelRunner by extracting the KV cache initialization logic into a new KVCacheInitializerMixin. This is a good change that improves code organization and makes GPUModelRunner easier to read and maintain. The implementation uses a Protocol for type-safe access to the GPUModelRunner instance from the mixin, which is a solid approach. The refactoring appears to be done correctly, and I did not find any issues with the changes.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @heheda12345.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork